1. Technical Field
The present invention generally relates to power supply circuits and in particular to voltage regulated power supplies for semiconductor based electronics. More specifically, the present invention provides a switching regulator with reduced reverse recovery currents.
2. Description of the Related Art
Power processing is an important feature in most electrical equipment. Differences in voltage and current requirements for various applications have resulted in the design of specific power converters to meet these requirements. Generally, power conversion involves the process of converting alternating current (AC) to direct current (DC) where converting the DC of one voltage level to the DC of another voltage level or both.
Linear regulators and switching regulators are the two basic varieties of power or voltage regulated converters. Although the switching regulator is not new concept, this type of regulator was not made practical until fast-switching high voltage transistors, low loss ferrite materials, and metallic glass materials became available. These developments allowed for practical and reliable construction of switching regulators.
Significant differences between linear and switching regulators include efficiency, size, weight, thermal requirements, response time, and noise characteristics. Switching regulators are generally preferred over linear regulators.
Switching regulators are commonly classified as one of three basic topologies: (1) buck; (2) boost; and (3) buck-boost. These switching regulators may be constructed from various arrangements of a switch, a diode, an inductor, and a capacitor.
FIGS. 1A-1C illustrate schematic diagrams of basic switching regulators well known in the prior art. FIG. 1A depicts a boost converter; FIG. 1B illustrates a buck converter; and FIG. 1C depicts a buck-boost converter. All three of these switching regulators are well known in the prior art. Each of these regulators has a switch S1, inductor L1, diode D1, and capacitor C1. A load 10 is coupled in parallel with the capacitor C1.
Referring again to FIG. 1A, inductor L1, diode D1, and capacitor C1 are connected in series to supply voltage Vs in the boost converter. Diode D1 serves as the rectifier in this circuit. Load 10 is connected in parallel with capacitor C1. Point P is connected to the negative pole of supply voltage source Vs by switching element S1. Switching element S1 preferably is a transistor, such as a metal oxide semiconductor field effect transistor (MOSFET). By switching the MOSFET into alternating on and off states, the voltage across capacitor C1 and load 10 will be higher than supply voltage Vs. As a result, this switching regulator functions as a voltage increasing circuit.
Since capacitor C1 will be positively charged on the side or plate facing diode D1, diode D1 will be biased in the reversed direction each time switching element S1 is in the on-state. Before the diode D1 blocks in the reverse direction, a charge stored during its "on" state in the forward direction gives rise to a reverse recovery current during the diode reverse recovery time. "Reverse recovery time" is the time that it takes to turn off a forward-biased diode through dissipating the stored charge in the diode.
More specifically, a boost converter typically involves the output rectifier being turned off directly by the MOSFET switch without a limiting impedance in series with the MOSFET switch. As the MOSFET switch turns on, the inductor acts as a current source and the MOSFET switch shifts the current from the output rectifier through the MOSFET switch itself. As the current in the rectifier drops to zero, the rectifier does not immediately shut off because charge is stored in the rectifier device. During the reverse recovery time, the rectifier actually conducts in the reverse direction, resulting in a reverse recovery current. In a boost topology, the output rectifier is connected directly to an output capacitor. Consequently, when the MOSFET switch turns on and reverse recovers the rectifier, the MOSFET switch is effectively turning on into the capacitor. In such situations the MOSFET switch may be subjected to large currents, thus possibly damaging the MOSFET switch.
The amount of reverse recovery current that may occur during recovery of the rectifier may be limited by: (1) the stored charge in the rectifier, which is dependent on device characteristics and operating current and (2) the potential reverse recovery current, which is dependent on switch turn on rate, output voltage, and output impedance. Depending on the switching regulator requirement, reverse recovery current problems may be solved by employing a fast rectifier and a switch capable of absorbing the reverse recovery current. This solution works for low voltage applications, such as applications which employ less than fifty volts.
Many applications however may require switching regulators with a high voltage output, such as three-hundred-ninety volts. In such a situation, reverse recovery currents are still a problem that must be dealt with in constructing efficient and reliable switching regulators.
Therefore, it would be desirable to have a switching regulator in which a reverse recovery current may be reduced or eliminated to avoid damaging the switch.